Ground-based mobile robotic vehicles (also termed ground-mobile robotic vehicles) are used to perform tasks hazardous to human safety. For example, ground-mobile robotic vehicles are often used by police and military personnel to move and/or detonate explosive devices. Ground-mobile robotic vehicles are used to obtain surveillance video and audio in hazardous locations, such as a building in which a suspected criminal may be located. Ground-mobile robotic vehicles are also used to reconnoiter potentially contaminated areas using chemical, nuclear, and/or biological sensors. Additionally, military ground-mobile robotic vehicles are used for security and the application of lethal force in combat areas.
Such ground-mobile robotic vehicles typically comprise an operator control unit (OCU) and an unmanned ground vehicle (UGV). The UGV typically comprises a chassis with a drive system including motor-driven wheels and/or an articulating track, a manipulator arm with gripper, surveillance camera and microphone for capturing video and audio for transmission to the OCU, a loudspeaker for broadcast of sound transmitted from the OCU, a plurality of sensors, mission-specific tools and implements, a communication interface (either hardwired or wireless, as discussed below), and one or more antennas (if the communication interface is wireless) for transmitting signals to and receiving signals from the OCU. The OCU typically comprises one or more control input devices, such as a joystick and keyboard, a display screen and other status indicators, a loudspeaker for broadcast of sound transmitted from the UGV, a microphone for capturing audio for transmission to and broadcast at the UGV, a communication interface (either hardwired or wireless, as discussed below), and one or more antennas (if the communication interface is wireless) for transmitting signals to and receiving signals from the UGV.
Either a hardwired (“tethered”) or wireless (“non-tethered”) communication link may be provided between the OCU and the UGV. Command and control data, as well as public address (PA) audio to be broadcast over the UGV loudspeaker, are typically transmitted from the OCU to the UGV via the communication link. Video and audio captured by the UGV, as well as sensor and status data, are typically transmitted from the UGV to the OCU via the communication link. A hardwired communication link typically comprises a fiber optic cable in a hardened casing. Such a hardwired communication link provides a large amount of bandwidth for transmitting large amounts of data quickly between the OCU and UGV. However, a hardwired communication link limits the distance that the UGV may travel from the OCU. Additionally, it is possible that the cable may snag on an obstruction thus preventing movement of the OCU. A wireless communication link typically comprises one or more radio frequency (RF) transmitters and receivers at both the OCU and the UGV. A wireless communication link between the OCU and the UGV will typically enable the UGV to travel further from the OCU and eliminates any risk of snagging. However, existing wireless communication links also present problems, as will be discussed in detail below.
Ground-based mobile robotic vehicles deployed using a wireless communication link typically involve transmitting and receiving a suitable set of base band signals over the RF communications link to allow for tele-operation (i.e., remote control) or monitoring of autonomous behavior by remote equipment or human beings. Typically, the base band signals transmitted and received over the RF communications link for real-time tele-operations comprise command and control data from the OCU to the UGV and low-latency camera video from the UGV to the OCU. The data signal must have a sufficiently low bit error rate and latency to provide safe and robust mobile command and control. The video signal must be of sufficient quality and, at the same time, have a sufficiently low latency to provide safe and robust mobile command and control. In addition to data and video, audio signals are often utilized to provide enhanced operator awareness of the UGV environment and status of the UGV itself. Audio generated from a surveillance microphone located on the UGV provides an audio signal synchronized with the live camera video and is transported in the same RF channel to drive a loudspeaker located at the OCU. A public address audio channel is used to provide audio generated from an operator microphone at the OCU to drive a loudspeaker located at the UGV, providing bi-directional voice communications between an operator and personnel in the UGV operating environment (e.g., remote hostage negotiation and imminent explosive detonation safety warning announcements).
The RF carrier modulation schemes typically employed to transmit and receive the video, audio, and data base band signals are amplitude modulation (AM), frequency modulation (FM), spread spectrum modulations such as direct sequence spread spectrum (DSSS) and frequency hopping spread spectrum (FHSS), and wireless local area networks (WLAN). Each of these RF carrier modulation schemes has regulatory restrictions and performance deficiencies problematic for use in ground-mobile robotic vehicles.
In the United States, federal law prohibits the transmission of composite video by AM from a mobile platform. As such, FM systems are commonly used for video, audio, and data. However, FM receivers, particularly wide-band receivers required for live composite video, have serious deficiencies in overcoming distortion products produced by multi-path fade signals encountered when systems are operated inside and around man-made structures and naturally occurring solid objects, such as vegetation and significant terrain contour. Further exacerbating the problem with wide band FM in mobility systems are restrictions on radiated power coupled with the poor receiver sensitivity inherent in a 17.5 MHz (or wider) front-end pass-band, which is typically required for an FM system during operations to transmit the necessary base band information. The maximum transmission power limits couple with poor sensitivity to produce a limited system dynamic range. Typically, this system dynamic power range is +37 dBm (the maximum Federal Communications Commission (FCC) transmission power limit for mobile FM video) to −83 dBm (the smallest signal detectable by an FM video receiver).
Spread spectrum techniques, which also may be wide-band when used to transport video (typically 22 MHz channel bandwidth or more), have range performance issues when used at power levels allowed by federal law for non-federal law enforcement and other public safety organizations for this application (i.e., +30 dBm or less) and are spectrally inefficient. WLAN has poor range at legal power limits, and is prone to interference from other devices, due to federal regulatory restrictions which cluster these devices in small, shared bands of RF spectrum.
The high bandwidth requirement of these modulation methods used in RF video links do not allow for a high level of wireless robotic system performance due to decreased receiver sensitivity. These modulation methods are typically susceptible to fading and distortion caused by high multi-path RF environments. These high multi-path RF environments are commonplace in a tele-operated ground mobile robotics application. Multi-path fading and distortion is produced when the signal of interest and ghost images of this signal, the ghost images having bounced off of obstacles in their path to the receiver, are all detected by the receiver front end at the same time but arrive at different phase angles. In systems with no means to correct arrival time errors (time domain), such as FM, AM, and most WLAN architectures, the ghost images are detected as noise products and degrade the critical signal to noise ratio at the receiver. Unmanned mobility platforms typically experience Rayleigh and Ricean (probability distribution) signal fading limiting the mobility stand-off range from the OCU, which again is worsened by a high bandwidth requirement. Additionally, FM does not allow for forward error correction, which is desirable in these applications to correct errors caused by fading and distortion.